Foreign politicians and officials who took part in two G20 summit meetings in London in 2009 had their computers monitored and their phone calls intercepted on the instructions of their British government hosts, according to documents seen by the Guardian. Some delegates were tricked into using internet cafes which had been set up by British intelligence agencies to read their email traffic.

The revelation comes as Britain prepares to host another summit on Monday – for the G8 nations, all of whom attended the 2009 meetings which were the object of the systematic spying. It is likely to lead to some tension among visiting delegates who will want the prime minister to explain whether they were targets in 2009 and whether the exercise is to be repeated this week.

The disclosure raises new questions about the boundaries of surveillance by GCHQ and its American sister organisation, the National Security Agency, whose access to phone records and internet data has been defended as necessary in the fight against terrorism and serious crime. The G20 spying appears to have been organised for the more mundane purpose of securing an advantage in meetings. Named targets include long-standing allies such as South Africa and Turkey.

There have often been rumours of this kind of espionage at international conferences, but it is highly unusual for hard evidence to confirm it and spell out the detail. The evidence is contained in documents – classified as top secret – which were uncovered by the NSA whistleblower Edward Snowden and seen by the Guardian. They reveal that during G20 meetings in April and September 2009 GCHQ used what one document calls "ground-breaking intelligence capabilities" to intercept the communications of visiting delegations.

This included:

• Setting up internet cafes where they used an email interception programme and key-logging software to spy on delegates' use of computers;

• Penetrating the security on delegates' BlackBerrys to monitor their email messages and phone calls;

• Supplying 45 analysts with a live round-the-clock summary of who was phoning who at the summit;

• Targeting the Turkish finance minister and possibly 15 others in his party;

• Receiving reports from an NSA attempt to eavesdrop on the Russian leader, Dmitry Medvedev, as his phone calls passed through satellite links to Moscow.

The documents suggest that the operation was sanctioned in principle at a senior level in the government of the then prime minister, Gordon Brown, and that intelligence, including briefings for visiting delegates, was passed to British ministers.

A briefing paper dated 20 January 2009 records advice given by GCHQ officials to their director, Sir Iain Lobban, who was planning to meet the then foreign secretary, David Miliband. The officials summarised Brown's aims for the meeting of G20 heads of state due to begin on 2 April, which was attempting to deal with the economic aftermath of the 2008 banking crisis. The briefing paper added: "The GCHQ intent is to ensure that intelligence relevant to HMG's desired outcomes for its presidency of the G20 reaches customers at the right time and in a form which allows them to make full use of it." Two documents explicitly refer to the intelligence product being passed to "ministers".

One of the GCHQ documents. Photograph: Guardian

According to the material seen by the Guardian, GCHQ generated this product by attacking both the computers and the telephones of delegates.

One document refers to a tactic which was "used a lot in recent UK conference, eg G20". The tactic, which is identified by an internal codeword which the Guardian is not revealing, is defined in an internal glossary as "active collection against an email account that acquires mail messages without removing them from the remote server". A PowerPoint slide explains that this means "reading people's email before/as they do".

The same document also refers to GCHQ, MI6 and others setting up internet cafes which "were able to extract key logging info, providing creds for delegates, meaning we have sustained intelligence options against them even after conference has finished". This appears to be a reference to acquiring delegates' online login details.

Another document summarises a sustained campaign to penetrate South African computers, recording that they gained access to the network of their foreign ministry, "investigated phone lines used by High Commission in London" and "retrieved documents including briefings for South African delegates to G20 and G8 meetings". (South Africa is a member of the G20 group and has observer status at G8 meetings.)

Another excerpt from the GCHQ documents. Photograph: Guardian

A detailed report records the efforts of the NSA's intercept specialists at Menwith Hill in North Yorkshire to target and decode encrypted phone calls from London to Moscow which were made by the Russian president, Dmitry Medvedev, and other Russian delegates.

Other documents record apparently successful efforts to penetrate the security of BlackBerry smartphones: "New converged events capabilities against BlackBerry provided advance copies of G20 briefings to ministers … Diplomatic targets from all nations have an MO of using smartphones. Exploited this use at the G20 meetings last year."

The operation appears to have run for at least six months. One document records that in March 2009 – the month before the heads of state meeting – GCHQ was working on an official requirement to "deliver a live dynamically updating graph of telephony call records for target G20 delegates … and continuing until G20 (2 April)."

Another document records that when G20 finance ministers met in London in September, GCHQ again took advantage of the occasion to spy on delegates, identifying the Turkish finance minister, Mehmet Simsek, as a target and listing 15 other junior ministers and officials in his delegation as "possible targets". As with the other G20 spying, there is no suggestion that Simsek and his party were involved in any kind of criminal offence. The document explicitly records a political objective – "to establish Turkey's position on agreements from the April London summit" and their "willingness (or not) to co-operate with the rest of the G20 nations".

The September meeting of finance ministers was also the subject of a new technique to provide a live report on any telephone call made by delegates and to display all of the activity on a graphic which was projected on to the 15-sq-metre video wall of GCHQ's operations centre as well as on to the screens of 45 specialist analysts who were monitoring the delegates.

"For the first time, analysts had a live picture of who was talking to who that updated constantly and automatically," according to an internal review.

A second review implies that the analysts' findings were being relayed rapidly to British representatives in the G20 meetings, a negotiating advantage of which their allies and opposite numbers may not have been aware: "In a live situation such as this, intelligence received may be used to influence events on the ground taking place just minutes or hours later. This means that it is not sufficient to mine call records afterwards – real-time tip-off is essential."

In the week after the September meeting, a group of analysts sent an internal message to the GCHQ section which had organised this live monitoring: "Thank you very much for getting the application ready for the G20 finance meeting last weekend … The call records activity pilot was very successful and was well received as a current indicator of delegate activity …

"It proved useful to note which nation delegation was active during the moments before, during and after the summit. All in all, a very successful weekend with the delegation telephony plot."